In Japanese Patent Application Kokai No. 5-181088 the present inventors proposed a novel, polarization-insensitive optical isolator comprising a Faraday rotator positioned between a pair of birefringent diffraction grating elements. Known birefringent diffraction grating elements useful for the optical isolator are as follows. They are all diffraction grating polarizers. Each element of the present invention by itself does not function as, but is technically equivalent to, a polarizer.
(1) The element according to Japanese Patent Application Kokai No. 63-55501; an element in which a diffraction grating is formed by subjection of lithium niobate to proton ion exchange. The element has a problem of high-cost manufacture because of the expensive single crystal substrate of lithium niobate for the grating. Another problem is that the difficulty of precise control of the optical path difference for polarization obstructs the fabrication of the elements in a stable way with good reproducibility.
(2) The element according to Japanese Patent Application Kokai No. 2-156205; a polarizer having a dielectric layer at the bottoms of grooves formed at regular intervals on the principal surface of an optically anisotropic crystal plate. The polarizer can be made at low cost, but the difficulties in accurately controlling the depth of grooves and the thickness of the dielectric layer render it impossible to control the optical path difference for polarization with high precision. Consequently, as with (1), stable production with good reproducibility has not been attained. In addition, rough bottom surface of the grooves can cause scattering, leading to deteriorated characteristics.
The present invention, therefore, aims at solving the afore-described problems and providing diffraction grating elements, especially birefringent diffraction grating elements, which permit easy control of the optical path length, production scheme, and designing, exhibit high performance stability with time, and have reduced thickness and also providing processes for producing them, and further providing optical isolators, optical circulators, and optical switches using those elements, and processes for producing them.